perf(subscriptions): Increase load factor to 3 for subscriptions test

[lynnagara]

Got some small improvements from #2268. Let's try pushing this to 3.

[codecov-commenter]

Codecov Report

Merging #2271 (cd005e4) into master (b88c2e1) will decrease coverage by 0.40%.
The diff coverage is 66.66%.

❗ Current head cd005e4 differs from pull request most recent head 507de14. Consider uploading reports for the commit 507de14 to get more accurate results

@@            Coverage Diff             @@
##           master    #2271      +/-   ##
==========================================
- Coverage   93.06%   92.65%   -0.41%     
==========================================
  Files         553      553              
  Lines       25238    25239       +1     
==========================================
- Hits        23487    23386     -101     
- Misses       1751     1853     +102     

Impacted Files	Coverage Δ
setup.py	0.00% <0.00%> (ø)
snuba/admin/clickhouse/nodes.py	52.63% <ø> (ø)
snuba/subscriptions/executor_consumer.py	88.98% <ø> (ø)
snuba/cli/subscriptions_executor.py	64.28% <100.00%> (+1.32%)	⬆️
snuba/settings/__init__.py	93.61% <100.00%> (ø)
...nsactions_onpremise_fix_orderby_and_partitionby.py	32.00% <0.00%> (-52.00%)	⬇️
...ts/0010_groupedmessages_onpremise_compatibility.py	51.11% <0.00%> (-48.89%)	⬇️
tests/migrations/test_runner_individual.py	68.14% <0.00%> (-31.86%)	⬇️
...ns/snuba_migrations/events/0014_backfill_errors.py	92.59% <0.00%> (-3.71%)	⬇️
snuba/migrations/table_engines.py	96.29% <0.00%> (-2.47%)	⬇️
... and 1 more

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